838 THE DISPERSION OF SPECIES BY MEANS OF FRUITS AND SEEDS. 



Kitaibelia of the family Malvaceae, Alstrcemeria amongst the Liliacese, several 

 Acanthaceae, meluding the Acanthus mollis (see figs. 459' and 459^), which Goethe 

 has made familiar to us, the wonderful parasite, Lathrcea clandestina, and many 

 Euphorbiaceae (e.g. Euphorbia, Hura, Hycenanthe, Mercurialis, Ricinus, see figs. 

 459" and 459'"). In all these plants the frait-valves are comparatively short, and 

 the spiral torsion is therefore less clearly manifested. The impulse given to the 

 seeds by the twisting of the valves is supplemented by various other contrivances 

 wliich cannot here be described, and, as a matter of fact, the range of projection in 

 this group of sling-fruits is wide as compared with that of others. 



A peculiar form of sling-fruit is found in several of the Diosmaceae, Rutaceae, 

 and Zygophyllacea3. In these plants a complete separation of the hard from the 

 soft layer takes place. When the seeds are ripe the external soft layer dries, splits 

 along the ventral suture, and contracts strongly. In consequence of this contraction 

 the hard internal layer, which is in the form of a case inclosing the seeds, is forced 

 out of the slit. As soon as the hard case is thus set at liberty its two lateral walls 

 part asunder, assume the shape of the screw of a steamer, and eject the seeds to a 

 distance. Similar processes occur in the genus Collomia of the family Polemoniaceae, 

 but in this instance it is not the soft outer layer of the valves but the calyx, which, 

 on drying, exercises pressure on the inclosed case, and the latter, which is extruded, 

 is not the hard layer only but the entire dry capsule. The liberation of the case is 

 here materially assisted by the circumstance that the three valves of the capsule 

 disunite at a time when they are still surrounded by the calyx, and hence exert a 

 counter-pressure upon the calyx. When once the capsule is freed from the grasp of 

 tlie calyx, its valves diverge still more widely from one another and eject their 

 seeds. In Eschscholtzia also the entire fruit is jerked off the receptacle, but here 

 the phenomenon depends on the fact that the two valves of the siliquose fruit attain 

 to a high degree of tension on desiccation and tend to curve outwards. When the 

 tension has reached a sufficient pitch to sever the connection between the fruit- 

 valves and the receptacle, the whole fruit is shot away from the stalk in a curve. 

 In the Stoi-k's-bill {Erodium, see vol. i. p. 619, figs. 147' and 147*), and in several 

 Umbellifers (e.g. Scandix), the entire fruit is not thrown off, but the constituent 

 parts of the fruit with their tightly inclosed seeds are jerked away from the central 

 axis. 



This cursory survey is sufficient to give an idea of the great variety existing 

 amongst the sling type of fruit. Of course the fruits in question are always placed 

 in such a position as to render a free flight of the seeds possible. In every case 

 where the fruits before ejecting their seeds or being themselves jerked away are 

 for any reason hidden under foliage-leaves, or are borne by stalks which bend 

 downwards, as in the Wood-sorrel and the Violet (see figs. 458^ and 458''), the 

 stalks straighten out just before expulsion takes place and lift the fruits up above 

 the leaves. In most instances the angle of projection is 45°, and, as is well known, 

 the greatest range of flight is thus attained. The ejected seeds are spherical, oval, 

 bean-shaped, or lenticular. In the last case they are expelled in such a manner as 



